The present invention relates to a random number generating circuit.
An oscillator of a random number generating circuit is comprised of a plurality of logic gate circuits which are connected in series between an input terminal and an output terminal. One output signal of the logic gate circuits is supplied to the input of the oscillator through a feedback line.
A frequency of the oscillator is based on both the logic gate circuits and a delay time caused by conductive lines in the oscillator. The delay time tends to change as a result of environmental temperature variations.
An example is disclosed in Japanese Laid Patent Publication: HEI 10-51276, published on Feb. 20, 1998.
However, in a circuit such as that is disclosed in the noted publication, a large number of the serially connected logic gate circuits is needed to obtain random numbers in a reliable manner. That is, the scale of the oscillator is large.
The object of the invention is to provide a random number generating circuit which is capable of reliable generating random number without increasing the scale of the oscillator.
To achieve the object, according to one aspect of the invention, in a random number generating circuit, a resistive element is arranged between a power supply line which supplies a power voltage and one of the logic gate circuits. An insulating layer is arranged between the resistive element and a conductive line. That is, a capacitor is formed between the resistive element and the conductive line and the random numbers are based on a capacitance of the capacitor.
According to the present invention, a degree of randomness of the random numbers which are generated from the random number generating circuit is improved without increasing the scale of the oscillator.